A grain-oriented electrical steel sheet is mainly used for a transformer core material for electric power and thus is required to be low in core loss. In a manufacturing method of a grain-oriented electrical steel sheet, a cold-rolled steel sheet having a final sheet thickness is subjected to decarburization annealing, and then is subjected to finish annealing aimed at secondary recrystallization and purification, and then is subjected to a process of forming a coating film on the steel sheet surface. The grain-oriented electrical steel sheet obtained in this manner is composed of a Si containing steel sheet having a sharp (110)[001] texture (Goss orientation) and a several micron inorganic coating film formed on the surface. The steel sheet has the Goss orientation, which is an essential condition for achieving a low core loss property of the grain-oriented electrical steel sheet, and for making this structure, grain growth called secondary recrystallization in which Goss oriented grains selectively grow during finish annealing is used.
For stably causing the secondary recrystallization, in the grain-oriented electrical steel sheet, fine precipitates in the steel called inhibitors are used. The inhibitor suppresses the grain growth in a low-temperature portion during finish annealing and at a certain temperature or higher, loses its pinning effect by decomposition or coarsening to cause the secondary recrystallization, and sulfide or nitride is generally used. For obtaining the desirable structure, it is necessary to keep the inhibitor up to a certain temperature, and if being sulfide, a sulfur component partial pressure in the finish annealing is controlled, and if being nitride, a nitrogen partial pressure is controlled or the like, and thereby the object of the desirable structure is accomplished. Sulfide and nitride used as the inhibitor are needed for the secondary recrystallization to occur in the middle of increasing the temperature during the finish annealing, but when they are retained in a product, they significantly deteriorate a core loss of the product. In order to remove an effect of sulfide and nitride from the steel sheet, after the secondary recrystallization is completed, the steel sheet is retained for a long time in pure hydrogen at around 1200° C. This is referred to as purification annealing. Thus, in the purification annealing, the steel sheet is in a state of being retained at a high temperature during the finish annealing.
On the other hand, the coating film of the grain-oriented electrical steel sheet is composed of a glass coating film and a secondary coating film, and by tension that these coating films apply to the steel sheet, a magnetic domain control effect is obtained and the low-core loss property is improved. As described in Patent Literature 1, if this tension is high, a core loss improving effect is high, and thus the secondary coating film in particular is required to have capability of generating high tension.
Generally, at the time of finish annealing, SiO2 in the steel sheet and MgO of an annealing separating agent main component react and thereby the glass coating film is formed on the steel sheet. The glass coating film has two functions. As the first function, the glass coating film tightly adheres to the steel sheet and the glass coating film itself has an effect of applying tension to the steel sheet and works as an intermediate layer to secure adhesiveness to the steel sheet when the secondary coating film to be formed in a process after the finish annealing is formed. When the adhesiveness of the glass coating film is good, the secondary coating film to generate high tension can be formed, and thus by the higher magnetic domain control effect, the low core loss can be achieved. Further, as the second function, the glass coating film has a function of preventing an excessive reduction in strength by the inhibitor during the finish annealing and stabilizing the secondary recrystallization. Thus, in order to stably manufacture a grain-oriented electrical steel sheet having a good magnetic property, the glass coating film having good adhesiveness to the steel sheet is required to be formed.
In order to improve the adhesiveness between the glass coating film and the steel sheet in the grain-oriented electrical steel sheet, it is necessary to optimize an interface structure between the glass coating film and the steel sheet. However, in a conventional grain-oriented electrical steel sheet, the sufficient adhesiveness is not necessarily secured when tension higher than ever before is desired to be applied, or the like.